1. Field of the Invention
This invention relates to a semiconductor light emitting device and its manufacturing method, especially suitable for application to a semiconductor light emitting device having a buried ridge structure and using nitride III-V compound semiconductors, and to fabrication thereof.
2. Description of the Related Art
Nitride III-V compound semiconductors represented by gallium nitride (GaN) are hopeful materials of light emitting devices capable of emitting light over a wide range from the green to blue and further to ultraviolet range, high-frequency electronic devices and anti-environmental electronic devices, for example. Particularly, since light emitting diodes using nitride III-V compound semiconductors were brought into practical use, nitride III-V compound semiconductors has been remarked largely. Realization of semiconductor lasers using nitride III-V compound semiconductors was also reported, and their application to light sources of optical disc apparatuses are hopefully expected.
A conventional GaN compound semiconductor laser is explained here. In the conventional GaN compound semiconductor laser, sequentially stacked by low-temperature growth on a c-plane sapphire substrate via a first GaN buffer layer are an n-type GaN contact layer, n-type AlGaN cladding layer, active layer, p-type AlGaN cladding layer and p-type GaN contact layer. The active layer has a single quantum well structure or a multi quantum well structure including a GaN layer as its emission layer. The upper-lying part of the p-type AlGaN cladding layer and the p-type GaN contact layer has a predetermined ridge stripe configuration extending in one direction. The upper-lying part of the n-type GaN contact layer, n-type AlGaN cladding layer, active layer and lower-lying part of the p-type AlGaN cladding layer has a predetermined mesa I configuration extending in parallel with the extending direction of the ridge stripe portion. Formed on the p-type GaN contact layer is a p-side electrode such as Ni/Pt/Au electrode or Ni/Au electrode in ohmic contact therewith, and formed on the n-type GaN contact layer in a location near the mesa portion is an n-side electrode such as Ti/Al/Pt/Au electrode in ohmic contact therewith.
In the conventional GaN compound semiconductor laser having the above-summarized structure, the upper-lying part of the p-type AlGaN cladding layer and the p-type GaN contact layer are patterned in to a ridge stripe configuration to restrict the current path, thereby to reduce the operation current and to control transverse modes by using a difference in effective refractive index between the ridge stripe portion and its opposite adjacent portions.
The conventional GaN compound semiconductor laser having the above-explained structure is manufactured as follows. That is, the first GaN buffer layer is grown on the c-plane sapphire substrate under a low temperature by metal organic chemical vapor deposition (MOCVD). Subsequently, by MOCVD, the second GaN buffer layer, n-type GaN contact layer, n-type AlGaN cladding layer, active layer, p-type AlGaN cladding layer and p-type GaN contact layer are sequentially grown on the first GaN buffer layer.
Then, after making a predetermined stripe-shaped mask extending in one direction on the p-type GaN contact layer, etching is conducted by reactive ion etching using the mask to the depth reaching an intermediate depth of the p-type AlGaN cladding layer to form the ridge stripe portion. Then, the mask is removed. After that, a predetermined stripe-shaped mask extending in one direction is formed on the p-type GaN contact layer and areas of the p-type GaN contact layer on both sides of the ridge stripe portion, and etching is conducted by RIE using this mask to the depth reaching a half depth of the n-type GaN contact layer to make a groove. Then, after removing the mask, the p-side electrode is formed on the p-type GaN contact layer, and the n-side electrode is formed on the n-type GaN contact layer.
After that, through the step of cleaving the sapphire substrate, having formed the laser structure as explained above, into bars along the direction vertical of the extending direction of the ridge stripe portion, or the step of dry etching, opposite cavity edges are made. Thereafter, each bar is divided into chips by dicing or scribing. Through these steps, the intended GaN compound semiconductor laser is completed.
In the conventional GaN compound semiconductor laser shown above, although it is controlled in transverse mode by using a difference in effective refractive index between the ridge stripe portion and its opposite adjacent portions, it is not configured to bury a semiconductor layer on both sides of the ridge stripe portion, unlike a buried ridge type AlGaAs compound semiconductor laser or AlGaInP compound semiconductor laser. Therefore, the conventional GaN compound semiconductor laser involved the problems that the difficulty in controlling transverse refractive index made it difficult to stabilize transverse modes, and its low heat dissipation effect made it difficult to realize a high output power and a long lifetime. Additionally, since the laser maintained the uneven structure made by the ridge stripe portion, an electrode made thereon for contact was liable to break due to a level difference. For this and other reasons, reliability of the laser was liable to decrease. Taking account of these matters, it is strongly desired to bury in both sides of the ridge stripe portion with an appropriate material also in the GaN semiconductor laser.
It is therefore an object of the invention to provide a semiconductor light emitting device using nitride III-V compound semiconductors, which is capable of stabilizing transverse modes and realizing a high output power and a long lifetime, and to provide a manufacturing method thereof.
According to the first aspect of the invention, there is provided a semiconductor light emitting device using a nitride III-V compound semiconductor, comprising:
a first cladding layer of a first conduction type;
an active layer on the first cladding layer;
a second cladding layer of a second conduction type on the active layer;
a stripe portion formed in the second cladding layer; and
a buried layer burying opposite sides of the stripe portion,
the buried layer being made by non-selectively growing the buried layer on the second cladding layer under the existence of a mask on the stripe portion and thereafter selectively removing the buried layer from above the stripe portion by etching using the mask on the stripe portion as an etching stop layer.
According to the second aspect of the invention, there is provided a manufacturing method for manufacturing a semiconductor light emitting device using a nitride III-V compound semiconductor, comprising the steps of:
sequentially growing a first cladding layer of a first conduction type, an active layer and a second cladding layer of a second conduction type on a substrate;
forming a stripe portion in the second cladding layer;
non-selectively growing a buried layer on the second cladding layer under the presence of a mask on the stripe portion; and
selectively removing the buried layer from above the stripe portion by etching using the mask on the stripe portion as an etching stop layer.
In the present invention, from the viewpoint of ensuring good current blocking, the buried layer is typically of a first conduction type or undoped. In the invention, from the viewpoint of ensuring good control of transverse modes, the buried layer typically has a lower refractive index than the second cladding layer. If necessary, however, the buried layer may function to absorb light from the active layer.
In the present invention, the buried layer is typically made of a nitride III-V compound semiconductor and preferably made of AlGaN because it can produce a difference in refractive index in the transverse direction and can readily control the difference in refractive index by changing the Al composition.
In the present invention, the nitride III-V compound semiconductor contains at least one group III element selected from the group consisting of Ga, Al, In, B and Tl, and at least N with or without As or P as group V elements. Examples of such nitride III-V compound semiconductors are GaN, AlGaN, GaInN, AlGaInN, and so forth.
In the present invention, the semiconductor light emitting device typically has a first contact layer of the first conduction type between the first cladding layer and the substrate, and includes a second contact layer of the second conduction type on the second cladding layer. Additionally, the semiconductor light emitting device preferably includes a first optical guide layer between the first cladding layer and the active layer, and includes a second optical guide layer between the active layer and the second cladding layer.
In the present invention, the mask formed on the stripe portion is the mask used as the etching mask upon making the stripe portion in the second cladding layer. Typically used as the material of the mask is a dielectric material or an insulator, more specifically, silicon oxide (SiO2)or silicon nitride (SiN). The mask is removed before providing an electrode in contact with the second cladding layer or the second contact layer of the second conduction type thereon.
In the second aspect of the invention, from the viewpoint of preventing damages to the surface of the buried layer on both sides of the stripe portion upon removal of the buried layer from above the stripe portion, another mask having an aperture in a location corresponding to the stripe portion is preferably made on the buried layer so as to selectively remove the buried layer from above the stripe portion by etching using the mask on the stripe portion and the mask on the buried layer as an etching stop layer.
In the second aspect of the invention, from the viewpoint of ensuring good surface smoothness of the buried structure, the buried layer is selectively removed from above the stripe portion by etching using a difference in crystallographic property between the buried layer grown on the second cladding layer and the buried layer grown on the mask on the stripe portion. This is attained by chemical etching using a potassium hydroxide solution, for example, as the etchant. In this case, the potassium hydroxide solution is preferably heated to a predetermined temperature, such as approximately 60xc2x0 C.
In the present invention, by providing a coating film on the buried layer and conducting an etch-back process using the coating film and the mask on the stripe portion as an etching stop layer, the buried layer may be removed from above the stripe portion.
According to the invention having the above-summarized structure, since the buried layer is non-selectively grown on the second cladding layer under the existence of the mask on the stripe portion, and the buried layer located on the stripe portion is removed by etching using the mask on the stripe portion as an etching stop layer, the structure burying both sides of the stripe portion with the buried layer can be made with a sufficient reproducibility and stably while using materials difficult to selectively grow as the material of the buried layer. Therefore, room for selection of the material of the buried layer is expanded. Additionally, since the buried layer buries both sides of the stripe portion, the controllability of the transverse refractive index and the heat dissipation effect are improved.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.